In the inking units of offset presses, ink consumption is adjusted over ductor roller length by duct blades or metering elements as the ductor roller rotates in adjustable angular steps while the press is running.
A separate motor, transmission and control electronics are usually provided to drive the ductor roller of large offset presses. In small offset presses, however, the drive is often mechanical. For example, a shaft driven by the press drive may have a crankpin and a drive rod pivotally connected thereto for providing oscillatory movements which are correspondingly speed-dependent. The drive rod has its other end pivotally connected to a ratchet drive disposed on the input journal of the ductor roller and thus drives the ductor roller in angular steps. Examples of this are known in the prior art from DE-PS 1,000,400 and DE-Gbm 6,628,071, for example.
With arrangements of the foregoing type, when the press stops the ductor roller also stops. As a consequence, the ink, depending upon its thickness or thinness, would in time spill out through the gap between the duct blade or metering element and the ductor roller and thus soil parts of the press below. This problem can be obviated, however, if the ductor roller continues to be driven when the main press cylinders are stationary. A constant ink film is thus produced on the ductor roller so that spillage is prevented.
In large offset presses having an independent ductor roller drive, the drive merely needs to remain energized during press stoppages in order to continue to rotate the ductor in steps, for example. However, press stoppages must be covered by an independent drive in the case of small offset presses in which the ductor roller drive is derived from the main drive. Such an arrangement is disclosed, for example, in U.S. Pat. No. 4,007,683. As disclosed here, in the event of a press stoppage, an auxiliary drive in the form of a separate motor continues to drive the ductor roller. The auxiliary drive is connected via a ratchet wheel to the drive journal of the ductor roller so that when the press is running and the auxiliary drive is inoperative, the auxiliary drive is automatically cut out of operation. It is also known in the prior art, in cases where there are a number of printing units, to arrange for a number of ductor rollers to be driven by a single auxiliary motor, for example, by means of flexible shafts, when the press is not running.
A disadvantage of such solutions is that auxiliary motors must be provided just to drive the ductor roller during press stoppages. This runs directly contrary to one of the primary features of small offset presses wherein the outlay on drive technology is much less because the ductor roller is driven directly by rotation of the press.